1. Technical Field
The present disclosure relates generally to the field of subsurface exploration and, more particularly, to techniques for determining subsurface parameters and well placement.
2. Background Information
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the subject matter described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, not as admissions of prior art.
Logging tools have long been used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the borehole and the fluids in the formations. Common logging tools include electromagnetic tools, nuclear tools, acoustic tools, and nuclear magnetic resonance (NMR) tools, though various other types of tools for evaluating formation properties are also available.
Early logging tools were run into a wellbore on a wireline cable after the wellbore had been drilled. Modern versions of such wireline tools are still used extensively. However, as the demand for information while drilling a borehole continued to increase, measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools have since been developed. MWD tools typically provide drilling parameter information such as weight on the bit, torque, temperature, pressure, direction, and inclination. LWD tools typically provide formation evaluation measurements such as resistivity, porosity, NMR distributions, and so forth. MWD and LWD tools often have characteristics common to wireline tools (e.g., transmitting and receiving antennas, sensors, etc.), but MWD and LWD tools are designed and constructed to endure and operate in the harsh environment of drilling.
Electromagnetic (EM) logging tools have been employed in the field of subsurface exploration for many years. These types of logging tools or instruments usually include an elongated support equipped with antennas that are operable as sources (transmitters) or sensors (receivers). The antennas on these tools are generally formed as loops or coils of conductive wires, and may be configured to have dipole moments that are axial (parallel with tool axis), transverse (perpendicular to the tool axis), or tilted (neither parallel nor perpendicular to the tool axis). In operation, a transmitter antenna is energized by an alternating current to emit EM energy through the borehole fluid (“mud”) and into the surrounding formation or formations. As used herein, “formation” may refer to a single layer or may include multiple layers. The emitted energy interacts with the borehole and formation to produce signals that are detected and measured by one or more receiver antennas. These detected signals reflect the interactions with the mud and the formation. The measurements are also affected by mud filtrate invasion that changes the properties of the rock near the wellbore. By processing the detected signal data, a log or profile of the formation and/or borehole properties is determined.
The processing of the measured subsurface parameters is done through a process known as an inversion technique (usually referred to as an “inversion”). In general, inversion processing includes making an initial estimate or model of the geometry and properties of the earth formations surrounding the well logging instrument. The initial model parameters may be derived in various ways known in the art. An expected logging instrument response is calculated based on the initial model. The calculated response is then compared with the measured response of the logging instrument. Differences between the calculated response and the measured response are used to adjust the parameters of the initial model, and the adjusted model is used to again calculate an expected response of the well logging instrument. The expected response for the adjusted model is compared with the measured instrument response, and any difference between them is used to again adjust the model. This process is generally repeated until the differences between the expected response and the measured response fall below a pre-selected threshold. By way of example, commonly-owned U.S. Pat. No. 6,594,584 describes some modern inversion techniques and is incorporated herein by reference in its entirety.
Using resistivity measurements made by LWD/MWD tools for well placement is commonly referred as in the industry as “geosteering.” In geosteering, estimation of the borehole position in real-time with respect to known geological markers is performed through correlation of resistivity log features in order to make steering decisions. The accuracy of such steering decisions often depends, at least in part, on the inversion technique used. Accordingly, the present disclosure describes novel inversion techniques that provide substantially real-time interpretation for well placement.